Hydromermis haggardi n. sp. (Nematoda: Mermithidae) is described from the hemocoel of chironomid imagos eclosing from Lake Alice Bog in northern Minnesota in June and July 2007. The species is distinguished from the other 27 described members of the genus by the subventral mouth, tulip shape to anterior end of esophagus, colorless vesicular trophosome, single nonbifurcated spicule, short body lengths of both males and females, structure of the S-shaped vagina, decrease in body width at vulva, brevity of postparasitic stage, and the greater development of a posterior ventral protractor muscle in the males. An anomalous adult male bearing large oval structures in the posterior testis is reported. Evidence is presented for the developmental interaction of the new species with Lanceimermis palustris Johnson and Kleve, 2004.